1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a solid polymer type fuel cell.
2. Description of the Related Art
A fuel cell has a pair of electrodes communicating via an electrolyte. One of the electrodes is supplied with fuel and the other is supplied with an oxidant for electrochemically reacting with the fuel, whereby chemical energy is directly converted to electric energy. More specifically, hydrogen gas that is fuel for a fuel cell is supplied to one electrode (anode), and oxygen that is an oxidant is supplied to the other electrode (cathode) so as to effect a reaction represented by the following Chemical Formulae (1) and (2) in the respective electrodes, whereby an electromotive force is generated.Anode: H2→2H++2e−  (1)Cathode: 2H++2e−+(1/2)O2→H2O  (2)
In recent years, as a fuel cell from which a high output is obtained, a solid polymer type fuel cell using a solid polymer electrolyte membrane as an electrolyte is attracting attention. For example, FIG. 8 is a cross-sectional view schematically showing a structure of main portions of a conventional fuel cell. In FIG. 8, a fuel cell 101 includes an electrolyte membrane 102, an anode 103 that is in contact with one surface of the electrolyte membrane 102 and is supplied with hydrogen (fuel), and a cathode 104 that is in contact with the other surface of the electrolyte membrane 102 and is supplied with air containing oxygen (oxidant). The anode 103 includes an anode catalyst layer 105 and a fuel-side gas diffusion layer 106 (e.g. carbon paper) that is in contact with the anode catalyst layer 105 to disperse hydrogen. The cathode 104 also includes an cathode catalyst layer 107 and an air-side gas diffusion layer 108 (e.g. carbon paper) that is in contact with the cathode catalyst layer 107 to disperse air. Furthermore, the fuel cell 101 includes fuel-side bipolar plates 110 made of carbon having a fuel channel 109 that is in contact with the fuel-side gas diffusion layer 106 to supply hydrogen to the anode 103, and air-side bipolar plates 112 made of carbon having an air channel 111 that is in contact with the air-side gas diffusion layer 108 to supply air to the cathode 104.
The electrolyte membrane 102 is made of, for example, a perfluorosulfonic acid polymer that is a solid polymer electrolyte membrane. One side of the electrolyte membrane 102 is coated with the anode catalyst layer 105 mixed with, for example, a platinum-ruthenium alloy catalyst, and the other side thereof is coated with the cathode catalyst layer 107 mixed with, for example, a platinum catalyst. In addition, a fuel gasket 113 for preventing leakage of hydrogen is disposed between the electrolyte membrane 102 and the fuel-side bipolar plates 110 around the anode 103. An air gasket 114 for preventing leakage of air is disposed between the electrolyte membrane 102 and the air-side bipolar plates 112 around the cathode 104.
The fuel cell 101 with the above-mentioned structure is produced as follows. First, carbon black powder with a platinum-ruthenium alloy (e.g. atomic ratio between platinum and ruthenium is 1:1) adhering to the surface thereof is mixed with an alcohol solution of a solid polymer electrolyte of perfluorosulfonic acid type to prepare an ink for an anode catalyst layer. Carbon black powder with platinum adhering to the surface thereof is also mixed with an alcohol solution of a solid polymer electrolyte of perfluorosulfonic acid type to prepare an ink for a cathode catalyst layer. The ink for the anode catalyst layer and the ink for the cathode catalyst layer are applied to two polytetrafluoroethylene sheets (hereinafter, referred to as “PTFE sheets”) by screen printing, respectively. The PTFE sheet coated with the ink for the anode catalyst layer and the PTFE sheet coated with the ink for the cathode catalyst layer are heated to dry each ink. Thereafter, the electrolyte membrane 102 is sandwiched between the two PTFE sheets so that the dried ink for the anode catalyst layer and the dried ink for the cathode catalyst layer come into contact with the respective sides of the electrolyte membrane 102 that is a perfluorosulfonic acid electrolyte membrane (e.g., Nafion film (Trade Name), produced by Dupont). The electrolyte membrane 102 in this state is subjected to hot press in the vicinity of a glass transition temperature of the solid polymer electrolyte membrane, whereby the ink for the anode catalyst layer and the ink for the cathode catalyst layer are heat-sealed to the electrolyte membrane 102. Then, the two PTFE sheets are removed. Consequently, as shown in FIG. 9, the ink for the anode catalyst layer remains as the anode catalyst layer 105 on one side of the electrolyte membrane 102, and the ink for the cathode catalyst layer remains as the cathode catalyst layer 107 on the other side of the electrolyte membrane 102.
Thereafter, the fuel-side gas diffusion layer 106 is overlapped with the anode catalyst layer 105. The fuel gasket 113 is disposed around the anode catalyst layer 105. The fuel-side bipolar plates 110 are overlapped with the fuel-side gas diffusion layer 106 and the fuel gasket 113. Similarly, the air-side gas diffusion layer 108 is overlapped with the cathode catalyst layer 107, and the air gasket 114 is disposed around the cathode catalyst layer 107. The air-side bipolar plates 112 are overlapped with the air-side gas diffusion layer 108 and the air gasket 114. Accordingly, the fuel cell 101 is produced.
Next, an operation of the fuel cell 101 will be described. FIG. 10 is a view schematically illustrating the state in the electrolyte membrane 102 during generation of electric power in a conventional fuel cell. In FIGS. 8 and 10, the anode catalyst layer 105 applied to the electrolyte membrane 102 is supplied with hydrogen gas from the fuel channel 109 via the fuel-side gas diffusion layer 106. The cathode catalyst layer 107 applied to the surface of the electrolyte membrane 102 opposed to the anode catalyst layer 105 is supplied with air from the air channel 111 via the air-side gas diffusion layer 108. When an external load 115 is connected to the anode 103 via the fuel-side bipolar plates 110, and to the cathode 104 via the air-side bipolar plates 112, a hydrogen gas performs the reaction represented by the above-mentioned Formula (1) in the anode catalyst layer 105, due to the platinum-ruthenium alloy of a metal catalyst, and hydrogen ions (H+) are supplied to the electrolyte membrane 102. Electrons (e) generated together with hydrogen ions due to this reaction pass through the external load 115 to reach the cathode 104. The electrolyte membrane 102 contains sufficient water, so that hydrogen ions generated on the side of the anode 103 are hydrated to flow to the cathode 104 while being accompanied by water, and performs the reaction represented by the above-mentioned Formula (2) with electrons and oxygen to generate water. The reaction operation is repeated to allow electrons to move, whereby a current flows to the external load 115.
For example, in the case where the fuel cell 101 is kept at 80° C., the fuel channel 109 is supplied with humidified hydrogen by a bubbler (not shown) kept at 70° C., and the air channel 111 is supplied with humidified air by a bubbler kept at 70° C. (i.e., in the case of a highly humidified state), the following results are obtained: the fuel cell 101 has 0.58 V/unit cell at a current density of 0.5 A/cm2. Furthermore, in the case where hydrogen and air are similarly supplied by a bubbler kept at 70° C. with 100 ppm of carbon monoxide contained in the hydrogen, the following results are obtained: the fuel cell 101 has 0.50 V/unit cell at a current density of 0.5 A/cm2. Herein, it is assumed that carbon monoxide is contained in fuel (hydrogen), as described below. Furthermore, in the case where humidified hydrogen and air are supplied to the fuel cell 101 kept at 80° C. by a bubbler kept at 60° C. (i.e., in the case of a low humidified state), the following results are obtained: the fuel cell 101 has 0.45 V/unit cell at a current density of 0.5 A/cm2.
Thus, as described above, hydrogen ions move accompanied by water in the electrolyte membrane 102, when moving from the anode 103 to the cathode 104. Water is also generated by the reaction of Formula (2) in the cathode 104. Therefore, water is likely to be accumulated in the cathode 104, whereas the anode 103 is likely to be dried due to the movement of water.
When a gradient of a water content is formed due to the movement of water between the anode 103 and the cathode 104, a force of water moving from the cathode 104 to the anode 103 is formed. However, the anode catalyst layer 105 is formed on substantially the entire surface of the electrolyte membrane 102, and the cathode catalyst layer 107 is formed on substantially the entire surface of the electrolyte membrane 102 opposite the anode catalyst layer 105, so that hydrogen ions are hydrated in the electrolyte membrane 102 and move from the anode 103 to the cathode 104, thereby preventing the movement of water from the cathode 104 to the anode 103 due to the gradient of the water content. Furthermore, water accumulated in the cathode 104 passes through the cathode catalyst layer 107 or the air-side gas diffusion layer 108 to be partially discharged to the outside of the fuel cell 101. The cathode 104 is dense, so that discharge of water is insufficient. As a result, water is likely to be accumulated in the cathode 104, and the anode 103 is likely to be dried.
In the case of setting a highly humidified state so as to prevent the anode 103 from being dried, accumulation of water in the cathode 104 is accelerated, and water enters pores in the cathode catalyst layer 107 and the air-side gas diffusion layer 108. Therefore, sufficient air is not supplied to the cathode catalyst layer 107, and the reaction in the cathode 104 is not smoothly effected.
Furthermore, in the case of setting a low humidified state so as to prevent accumulation of water in the cathode 104, drying of the anode 103 is accelerated, and generated hydrogen ions are hydrated and cannot move to the cathode 104. This increases an ion-conduction resistance, and hydrogen ions cannot be sufficiently supplied to the cathode 104.
There is also a method in which movement of water from the cathode 104 to the anode 103 is activated by making the electrolyte membrane 102 thin and increasing the gradient of the water content between the anode 103 and the cathode 104. When the electrolyte membrane 102 becomes thin, there arises a problem of increasing the danger that the electrolyte membrane 102 is damaged.
Furthermore, in general, hydrogen as fuel is generated from manufactured gas, methanol, or the like, so that hydrogen as fuel contains carbon monoxide or the like as a sub-product. Carbon monoxide in hydrogen adsorbs to platinum of a metal catalyst. Therefore, when such fuel is used, the function of platinum as a catalyst is reduced. In order to prevent this, ruthenium is added to the anode catalyst layer 105, and a platinum-ruthenium alloy is used as a metal catalyst. However, as the added amount of ruthenium is increased, the amount of platinum is reduced accordingly. Therefore, in the case where hydrogen containing no carbon monoxide is used as fuel, it is not appropriate to use a catalyst containing a large amount of ruthenium as a metal catalyst. Thus, the composition of an appropriate metal catalyst is varied depending upon the composition of fuel, resulting in a problem in that the anode catalyst layer 105 cannot have sufficient performance, except for the case where a metal catalyst is used for the anode catalyst layer 105, and fuel allowing the metal catalyst to exhibit its function as a catalyst is supplied.